Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/848

 cylindrical pieces deeply channelled on one side indicate the presence of the ventral set common in all Plesiosauri.

Breast and Shoulder-girdle. — The large flat bones, which can belong only here, differ so greatly from those of known Plesiosauri, that the only ones the determination of which can be looked on as at all certain are the coracoids (Pl. XLI. fig. 5). These are joined firmly together by a very stout median border, and form a strong flattened beam lengthened transversely to the axis of the trunk, attaining in this direction a maximum diameter of 20 inches at the lower end of the glenoid cavity, and one of 18 inches at the upper end of this articular surface. Each coracoid, then, is an oblong plate having a very short sutural median border which ties it to its fellow, and a gently concave anterior border decreasing in thickness from the outer end (where it contributes to the scapular articulation ?) towards the middle, from which it again increases to its inner end, where the two coracoids meet in a remarkably strong non-articular projecting boss (a). Only a small piece of the natural posterior edge remains. It shows that this border curved inwards from behind the glenoid cavity towards the median line. How far the bone may have extended in this direction cannot be told ; but, from the inclination of its ventral and dorsal surfaces towards each other, the mesial extension backwards would seem not to have been great. The outer border of the coracoid is stout; it bears a large, hollow, oval articular surface (the glenoid cavity, fig. 6, a), and in front of this a smaller trihedral surface, presumably for articulation with the scapular (d). The dorsal (?) surface of the coracoid is slightly sinuous. The ventral surface has a deep hollow just behind and parallel to the anterior border of the bone. A stout ridge sweeping from the median boss on the anterior border outwards to the glenoid border, divides this hollow from another triangular hollow, which lies between this ridge and the median crest. Below the outer end of this transverse ridge the ventral surface is convex from before backwards, and nearly plane transversely.

Limbs. — Of the four principal limb-bones the left pair are nearly perfect ; my description is taken from these. One is rather shorter than the other ; and this I assume to be the humerus, because in those Plesiosaurs in which there is any difference in the lengths of the humerus and femur the former is the shorter bone. In our Plesiosaur the humerus (fig. 1) is also stouter than the femur. Its length is 20-1/2 inches. Its proximal half is very massive and of a subcylindrical figure, while the distal half is expanded and flattened. The preaxial border (c) is nearly a straight line, broken both at the middle and towards the distal end by a slight excursion. The postaxial border (d) in its proximal 12 inches is nearly parallel to the preaxial one ; but beyond this it sweeps backwards in a long curve and makes with the distal border a great wing, much larger relatively than that of typical Plesiosauri. The contour of the distal border (b) is formed by two slightly incurved lines which meet in a wide angle placed rather behind the axis of the bone. The postaxial division, which is much the longer, is, unfortunately, incomplete. This end of the